Add heatmap visualization mode for markers

[dmitry-zasypkin]

Heatmap sample code when using Leaflet library (https://leafletjs.com/):

	this._mapObject = L.map(mapDiv).setView([38.845210, 24.333653], 7);

	L.tileLayer('https://b.tile.openstreetmap.org/{z}/{x}/{y}.png', 
		{
			attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>',
			maxZoom: 18
		}
	).addTo(this._mapObject);

	this._markerData[this._markerData.length] = [lat1, long1, 1];
	...
	this._markerData[this._markerData.length] = [latN, longN, 1];
 
	var heat = L.heatLayer(this._markerData, {radius: 15}).addTo(this._mapObject);

Files needed to make that code work:
https://unpkg.com/[email protected]/dist/leaflet.css
https://unpkg.com/[email protected]/dist/leaflet.js
leaflet-heat.js (from https://github.com/Leaflet/Leaflet.heat)